diff --git a/README.md b/README.md
index d0ffef9de6..4a56304e16 100644
--- a/README.md
+++ b/README.md
@@ -66,8 +66,58 @@ from any mobile devices you ran it on) when done.
 [download-zuliprc]: https://zulip.com/api/api-keys
 
 
+### Tests
+
+You can run all our forms of tests with two commands:
+
+```
+$ flutter analyze
+$ flutter test
+```
+
+Both should always pass, with no errors or warnings of any kind.
+
+The `flutter analyze` command runs the Dart analyzer, which performs
+type-checking and linting.  The `flutter test` command runs our
+unit tests, located in the `test/` directory.
+
+Both commands accept a list of file or directory paths to operate
+only on those files, and other options.
+
+When editing in an IDE, the IDE should give you the exact same feedback
+as `flutter analyze` would.  When editing a test file, the IDE can also
+run individual tests for you.
+See [upstream docs on `flutter test`][flutter-cookbook-unit-tests].
+
+[flutter-cookbook-unit-tests]: https://docs.flutter.dev/cookbook/testing/unit/introduction
+
+
 ## Notes
 
+### Writing tests
+
+For unit tests, we use [the `checks` package][package-checks].
+This is a new package from the Dart team, currently in preview,
+which is [intended to replace][package-checks-migration] the
+old `matcher` package.
+
+This means that if you see example test code elsewhere that
+uses the `expect` function, we'd prefer to translate it into
+something in terms of `check`.  For help with that,
+see the [`package:checks` migration guide][package-checks-migration]
+and the package's [API docs][package-checks-api].
+
+Because `package:checks` is still in preview, the Dart team is
+open to feedback on the API to a degree that they won't be
+after it reaches 1.0.  So where we find rough edges, now is a
+good time to [report them as issues][dart-test-tracker].
+
+[package-checks]: https://pub.dev/packages/checks
+[package-checks-api]: https://pub.dev/documentation/checks/latest/checks/checks-library.html
+[package-checks-migration]: https://github.com/dart-lang/test/blob/master/pkgs/checks/doc/migrating_from_matcher.md
+[dart-test-tracker]: https://github.com/dart-lang/test/issues
+
+
 ### Editing API types
 
 We support Zulip Server 4.0 and later.  For API features added in
@@ -96,7 +146,7 @@ To update the version bounds:
 * Update the lower bounds at `environment` in `pubspec.yaml`
   to the new versions, as seen in `flutter --version`.
 * Run `flutter pub get`, which will update `pubspec.lock`.
-* Make a quick check that things work: `flutter analyze`,
+* Make a quick check that things work: `flutter analyze && flutter test`,
   and do a quick smoke-test of the app.
 * Commit and push the changes in `pubspec.yaml` and `pubspec.lock`.
 
diff --git a/lib/model/content.dart b/lib/model/content.dart
index 50d4a938e4..3858d3e414 100644
--- a/lib/model/content.dart
+++ b/lib/model/content.dart
@@ -2,8 +2,15 @@ import 'package:flutter/foundation.dart';
 import 'package:html/dom.dart' as dom;
 import 'package:html/parser.dart';
 
+// TODO: Implement ==/hashCode for all these classes where O(1), for testing/debugging
+//   (Skip them for classes containing lists.)
+// TODO: Implement toString for all these classes, for testing/debugging; or
+//   perhaps Diagnosticable instead?
+
+// This should have no subclasses except the ones defined in this file.
+// TODO mark as sealed: https://github.com/dart-lang/language/blob/a374667bc/accepted/future-releases/sealed-types/feature-specification.md
 @immutable
-class ContentNode {
+abstract class ContentNode {
   const ContentNode({this.debugHtmlNode});
 
   final dom.Node? debugHtmlNode;
@@ -28,6 +35,9 @@ class ZulipContent extends ContentNode {
   const ZulipContent({super.debugHtmlNode, required this.nodes});
 
   final List<BlockContentNode> nodes;
+
+  @override
+  String toString() => '${objectRuntimeType(this, 'ZulipContent')}($nodes)';
 }
 
 abstract class BlockContentNode extends ContentNode {
@@ -45,6 +55,14 @@ class UnimplementedBlockContentNode extends BlockContentNode
 // A `br` element.
 class LineBreakNode extends BlockContentNode {
   const LineBreakNode({super.debugHtmlNode});
+
+  @override
+  bool operator ==(Object other) {
+    return other is LineBreakNode;
+  }
+
+  @override
+  int get hashCode => 'LineBreakNode'.hashCode;
 }
 
 // A `p` element, or a place where the DOM tree logically wanted one.
@@ -63,6 +81,12 @@ class ParagraphNode extends BlockContentNode {
   final bool wasImplicit;
 
   final List<InlineContentNode> nodes;
+
+  // No == or hashCode overrides; don't want to walk through [nodes] in
+  // an operation that looks cheap.
+
+  @override
+  String toString() => '${objectRuntimeType(this, 'ParagraphNode')}(wasImplicit: $wasImplicit, $nodes)';
 }
 
 enum ListStyle { ordered, unordered }
@@ -122,6 +146,19 @@ class TextNode extends InlineContentNode {
   const TextNode(this.text, {super.debugHtmlNode});
 
   final String text;
+
+  @override
+  bool operator ==(Object other) {
+    return other is TextNode
+        && other.text == text;
+  }
+
+  @override
+  int get hashCode => Object.hash('TextNode', text);
+
+  // TODO encode unambiguously regardless of text contents
+  @override
+  String toString() => '${objectRuntimeType(this, 'TextNode')}(text: $text)';
 }
 
 class LineBreakInlineNode extends InlineContentNode {
diff --git a/pubspec.lock b/pubspec.lock
index cf8a0e1513..8ff1657685 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -121,6 +121,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.0.2"
+  checks:
+    dependency: "direct dev"
+    description:
+      name: checks
+      sha256: "028fb54c181e68edc3c891d7d45813937311e29c77609f513bb5025620637ffd"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.1"
   clock:
     dependency: transitive
     description:
@@ -153,6 +161,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.1"
+  coverage:
+    dependency: transitive
+    description:
+      name: coverage
+      sha256: "2fb815080e44a09b85e0f2ca8a820b15053982b2e714b59267719e8a9ff17097"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.3"
   crypto:
     dependency: transitive
     description:
@@ -371,6 +387,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.4"
+  node_preamble:
+    dependency: transitive
+    description:
+      name: node_preamble
+      sha256: "8ebdbaa3b96d5285d068f80772390d27c21e1fa10fb2df6627b1b9415043608d"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.0.1"
   package_config:
     dependency: transitive
     description:
@@ -419,6 +443,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.4.0"
+  shelf_packages_handler:
+    dependency: transitive
+    description:
+      name: shelf_packages_handler
+      sha256: aef74dc9195746a384843102142ab65b6a4735bb3beea791e63527b88cc83306
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.1"
+  shelf_static:
+    dependency: transitive
+    description:
+      name: shelf_static
+      sha256: e792b76b96a36d4a41b819da593aff4bdd413576b3ba6150df5d8d9996d2e74c
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.1"
   shelf_web_socket:
     dependency: transitive
     description:
@@ -448,6 +488,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.3.3"
+  source_map_stack_trace:
+    dependency: transitive
+    description:
+      name: source_map_stack_trace
+      sha256: "84cf769ad83aa6bb61e0aa5a18e53aea683395f196a6f39c4c881fb90ed4f7ae"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.1.1"
+  source_maps:
+    dependency: transitive
+    description:
+      name: source_maps
+      sha256: "708b3f6b97248e5781f493b765c3337db11c5d2c81c3094f10904bfa8004c703"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.10.12"
   source_span:
     dependency: transitive
     description:
@@ -496,6 +552,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.2.1"
+  test:
+    dependency: "direct dev"
+    description:
+      name: test
+      sha256: "5301f54eb6fe945daa99bc8df6ece3f88b5ceaa6f996f250efdaaf63e22886be"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.23.1"
   test_api:
     dependency: transitive
     description:
@@ -504,6 +568,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.4.18"
+  test_core:
+    dependency: transitive
+    description:
+      name: test_core
+      sha256: d2e9240594b409565524802b84b7b39341da36dd6fd8e1660b53ad928ec3e9af
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.4.24"
   timing:
     dependency: transitive
     description:
@@ -528,6 +600,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.4"
+  vm_service:
+    dependency: transitive
+    description:
+      name: vm_service
+      sha256: eb3cf3f45fc1500ae30481ac9ab788302fa5e8edc3f3eaddf183945ee93a8bf3
+      url: "https://pub.dev"
+    source: hosted
+    version: "11.2.0"
   watcher:
     dependency: transitive
     description:
@@ -544,6 +624,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.3.0"
+  webkit_inspection_protocol:
+    dependency: transitive
+    description:
+      name: webkit_inspection_protocol
+      sha256: "67d3a8b6c79e1987d19d848b0892e582dbb0c66c57cc1fef58a177dd2aa2823d"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.0"
   yaml:
     dependency: transitive
     description:
diff --git a/pubspec.yaml b/pubspec.yaml
index f5f9bd285f..6895ba6589 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -59,6 +59,8 @@ dev_dependencies:
 
   json_serializable: ^6.5.4
   build_runner: ^2.3.3
+  test: ^1.23.1
+  checks: ^0.2.1
 
 # For information on the generic Dart part of this file, see the
 # following page: https://dart.dev/tools/pub/pubspec
diff --git a/test/model/content_checks.dart b/test/model/content_checks.dart
new file mode 100644
index 0000000000..402c43ae1d
--- /dev/null
+++ b/test/model/content_checks.dart
@@ -0,0 +1,41 @@
+import 'package:checks/checks.dart';
+import 'package:zulip/model/content.dart';
+
+extension ContentNodeChecks on Subject<ContentNode> {
+  void equalsNode(ContentNode expected) {
+    if (expected is ZulipContent) {
+      isA<ZulipContent>()
+        .nodes.deepEquals(expected.nodes.map(
+          (e) => it()..isA<BlockContentNode>().equalsNode(e)));
+        // A shame we need the dynamic `isA` there.  This
+        // version hits a runtime type error:
+        //   .nodes.deepEquals(expected.nodes.map(
+        //     (e) => it<BlockContentNode>()..equalsNode(e)));
+        // and with `it()` with no type argument, it doesn't type-check.
+        // TODO: report that as API feedback on deepEquals
+    } else if (expected is ParagraphNode) {
+      isA<ParagraphNode>()
+        ..wasImplicit.equals(expected.wasImplicit)
+        ..nodes.deepEquals(expected.nodes.map(
+          (e) => it()..isA<InlineContentNode>().equalsNode(e)));
+    } else {
+      // TODO handle remaining ContentNode subclasses that lack structural ==
+      equals(expected);
+    }
+  }
+}
+
+extension ZulipContentChecks on Subject<ZulipContent> {
+  Subject<List<BlockContentNode>> get nodes => has((n) => n.nodes, 'nodes');
+}
+
+extension ParagraphNodeChecks on Subject<ParagraphNode> {
+  Subject<bool> get wasImplicit => has((n) => n.wasImplicit, 'wasImplicit');
+  Subject<List<InlineContentNode>> get nodes => has((n) => n.nodes, 'nodes');
+}
+
+extension TextNodeChecks on Subject<TextNode> {
+  Subject<String> get text => has((n) => n.text, 'text');
+}
+
+// TODO write similar extensions for the rest of the content node classes
diff --git a/test/model/content_test.dart b/test/model/content_test.dart
new file mode 100644
index 0000000000..75f33e8f4a
--- /dev/null
+++ b/test/model/content_test.dart
@@ -0,0 +1,24 @@
+import 'package:checks/checks.dart';
+import 'package:test/scaffolding.dart';
+import 'package:zulip/model/content.dart';
+
+import 'content_checks.dart';
+
+void main() {
+  test('parse a plain-text paragraph', () {
+    check(parseContent('<p>hello world</p>'))
+      .equalsNode(const ZulipContent(nodes: [
+        ParagraphNode(nodes: [TextNode('hello world')]),
+      ]));
+  });
+
+  test('parse two plain-text paragraphs', () {
+    check(parseContent('<p>hello</p><p>world</p>'))
+      .equalsNode(const ZulipContent(nodes: [
+        ParagraphNode(nodes: [TextNode('hello')]),
+        ParagraphNode(nodes: [TextNode('world')]),
+      ]));
+  });
+
+  // TODO write more tests for this code
+}